Add DB property "rocksdb.estimate-table-readers-mem"

Summary:
Add a DB Property "rocksdb.estimate-table-readers-mem" to return estimated memory usage by all loaded table readers, other than allocated from block cache.

Refactor the property codes to allow getting property from a version, with DB mutex not acquired.

Test Plan: Add several checks of this new property in existing codes for various cases.

Reviewers: yhchiang, ljin

Reviewed By: ljin

Subscribers: xjin, igor, leveldb

Differential Revision: https://reviews.facebook.net/D20733
Fix valgrind failure caused by recent checked-in.

Summary: Initialize un-initialized parameters

Test Plan: run the failed test (c_test)

Reviewers: yhchiang, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D21249
Minor: fix a format

Summary: A format fixing

Test Plan: N/A

Reviewers: ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D21255